1. Field of the Invention
The present invention relates generally to communication signal detection methods and apparatus, and more particularly to an improved method and apparatus for detecting complex signals transmitted with designations according to the V.29 constellation.
2. Description of the Prior Art
One of the ways in which data can be encoded for transmission is to designate an array of data points in a complex plane and then assign each of these points to represent a particular pattern of digital information. These data points can be described by either real and imaginary or phase and amplitude numbers. Each such array of data points is called a constellation and certain constellations have been recognized by CCITT and given designations such as V.27 ter, V.29, etc. Using this type of encoding scheme, the signal received at a receiving station can be related to the points in the constellation and identified with the original information transmitted. Optimum multi-level signal detection theory requires that the constellation signal "closest" to the received signal be selected as the most likely signal transmitted. "Closest" in the complex plane is defined as the constellation point in the set with the shortest magnitude, i.e., .sqroot.X.sup.2 +Y.sup.2, to the received signal. If the constellation is defined on a rectangular grid in the complex plane, then the minimum distance can be determined by simple and independent tests on the magnitude of the real and imaginary components of the received signal. If the constellation is arranged in a circular fashion, then the regions are normally defined in terms of polar coordinates rather than quadrature component definitions. If the receiver delivers quadrature components, a rectangular-to-polar transformation must be performed in order to determine the most likely signal transmitted. In the case of the digital detector, this rectangular-to-polar transformation requires a number of computations including multiplication operations which are costly in terms of digital multiplication.
Although there are numerous constellations that have been defined, the V.29 constellation is one that cannot be classified as a pure rectangular or pure angle type of constellation. Hence, an optimum detector for this constellation would normally be expected to be even more complex than detectors used for either the pure rectangular or pure angle type of constellations.